Common mode chokes (inductors) are four-terminal devices with different common mode and differential mode characteristics. Common mode chokes are used in electromagnetic interference (EMI) filters and other circuits to suppress unwanted high frequency noise without significantly affecting desired signals.
A typical configuration of a common mode choke includes a winding bobbin divided into three sections with a first outer section, a middle section and a second outer section. A first winding is wound about the first outer section of the bobbin. A second winding is wound about the second outer section of the bobbin. The middle section of the bobbin does not include a winding. The empty middle section creates spacing between the windings on the two outer sections. The spacing increases the leakage inductance between the two outer windings. In many applications, increasing the leakage inductance is desirable to suppress EMI differential noise. The leakage inductance can be further increased by increasing the distance between the two windings. However, increasing the spacing between the two outer windings is not always an option because the increased length of the bobbin may not be acceptable.